The present invention is directed to a method of detecting and monitoring a leak caused by a through wall crack in a high pressure fluid system and more specifically to a nuclear reactor piping system.
Nuclear piping systems are designed under code provisions that reflect a leak-before-break philosophy. Piping is typically austenitic stainless steel and past service history has shown that through wall cracks will occur without serious threat to system integrity; i.e., the pipe breaking. Thus, leak detection is an important method for detecting a fracture process in these piping systems.
In reactor systems where large volumes and flow rates are involved it is believed that detection of escaping fluids holds the most promise for improved sensitivity. Several present techniques now in use on reactor systems include flow detection, radio isotope gas detection, nuclear radiation detection, and pressure and temperature changes in a "collection" chamber. Methods which have been proposed for future use include acoustic noise monitoring and moisture sensitive foil which exhibits a conductivity change in the presence of moisture.
One form of acoustic monitoring which has been developed is acoustic emission. Usually acoustic emission is defined as the elastic waves generated when a material undergoes plastic deformation. Here when the structure is loaded, emissions occur in burst type pulses and it is possible to calculate source location with the use of several transducers which are affixed directly to the structure being monitored.
A definitive study in the application of acoustic emission to leak detection was made to J. D. Allison, O. A. Kupcis, and O. C. Irwin, "Detection of Leaks in CANDU Reactor Fuel Channels Using Acoustic Emission Monitoring," CWAPD-267 Westinghouse Canada Limited, Dec. 5, 1974. Here a transducer was in contact with the end fitting of each of the several pressure tubes in the reactor. Frequency analysis of the detected signal was conducted in the frequency range up to 1.0 MHz. As illustrated in FIG. 1, signals were analyzed on a frequency amplitude basis and the frequency spectra for the tubes indicated as 11, 12 and 13 were identified as leaking tubes as opposed to the nonflawed or sound tube represented by curve 14. In other words, this experiment indicated that by the use of acoustic emission in a frequency amplitude plot, the presence of a leak might be indicated by the frequency distribution or frequency spectra.
However, merely detecting a leak is not sufficient. It is desired to quantify the leak and provide an indication of the crack growth through the wall of the piping. This would be a significant aid in determining operating and shutdown repair criteria.